[tdavis]

Now make some other code use your "mock" without editing its source. Now assert on the calls made to "bar()," including arguments. Now make it raise an exception without writing a second class. And make it return something else without writing a third. Now patch a built-in. And a context manager. And a dictionary. When you're done with that, mock a class without writing a new class at all.

If you honestly think that sub-classing is equivalent to mocking/stubbing, you don't understand the purpose of the activity.

[jrockway]

I know libraries can do all those things, but I've never preferred it over the real thing. As an example, I might do something like this (not Python):

   my $bar = 0;
   class FakeFoo {
      sub bar { return $bar }
   }

   my $thing = ThingToTest->new( foo => FakeFoo->new );
   $foo = 42;
   is $thing->make_bar_two_bigger, 44;
   $foo = -10;
   is $thing->make_bar_two_bigger, -12;

Mock enthusiasts would write:

   my $fake_foo = Mock->new( Foo->class )->instance;
   $fake_foo->return_sequence( bar => [42, -10] );
   my $thing = ThingToTest->new( foo => $fake_foo );
   is $fake_foo->make_bar_two_bigger, 44;
   is $fake_foo->make_bar_two_bigger, -12;
   ok $fake_foo->ensure_exhausted('bar');

Yes, it's less lines, but I think it's harder to read because the values are being set up long before they're used.

[jrockway]

Well, this was typo-y. $foo in the first codeblock should be $bar, and $fake_foo->make_bar_two_bigger should be $thing->make_bar_two_bigger. More coffee is required :)